In a transmission electron microscope, a beam of high-energy electrons is directed toward a thin sample. Electrons in the beam interact with the sample as they pass through it and are collected below the sample. Some electrons pass through the sample relatively unhindered; others are deflected, absorbed, or lose energy. The electrons that pass through the sample carry information about the sample and produce a pattern on a detector. The pattern may correspond to, for example, an image of the sample, a diffraction pattern, or a spectrum of electron energy. Different imaging and analysis techniques use different characteristics of the transmitted electrons to form an image or to determine properties of the sample.
For example, information about the sample can be provided by measuring the energy absorbed as electrons pass though the sample. This technique is called “electron energy-loss spectroscopy” or EELS. An overview of EELS is provided by R. F. Egerton in “Electron energy-loss spectroscopy in the TEM,” Reports on Progress in Physics 72 (December 2008). Different materials in the sample cause electrons to lose different amounts of energy as they pass through. The electrons pass through a spectrometer to determine the energy loss by subtracting their existing energy from the electron energy in the original electron beam. EELS can determine not only which elements are present, but also their chemical states.
An EELS spectrometer typically includes one or more prisms that separate electrons by their energies in an energy-dispersive plane by deflecting the electrons by an amount that depends on the electron energy. An energy dispersive plane is a plane in which electrons having different energies are dispersed in a direction normal to the direction of the beam travel. The term “prism” as used herein means any device that disperses the electron beam depending on the energies of electrons in the beam. A prism can provide, for example, a magnetic or electric field perpendicular to the beam. For example, a portion of a spherical capacitor, a magnetic deflector, or a Wien filter can be used as a prism. The angular dispersion of the electrons depends on the strength of the magnetic or electric field in the prism and the energy of the electrons. A prism may comprise multiple elements. Beside a prism, an EELS spectrometer may also include an adjustable energy-selecting slit, typically positioned in or near the energy-dispersive plane, and imaging optics that may include a system of prisms and/or lenses and/or multipoles or combinations thereof, to form an electron image on a detector that records the image. The detector can be, for example, a charged coupled device or active pixel sensor and may include a row or a two-dimensional array of pixels. Projection optics positioned after the sample and before the spectrometer project electrons into the entrance aperture of the spectrometer.
There are several mechanisms by which electrons lose energy as they pass through a sample. The different mechanisms cause electrons to lose different amounts of energy and account for the shape of a typical energy loss graph or spectrum. FIGS. 1A and 1B are spectra that show in arbitrary units numbers of electrons detected at various energy loss values. The energy loss spectrum varies with the material present in the sample and so information about the sample can be inferred from the spectrum.
FIG. 1A shows the so-called “low-loss” region 100 of the energy loss spectrum, which is defined somewhat arbitrarily as regions of less than 100 eV. Electron losses in the low-loss region result primarily from inelastic interactions, such as phonon interactions, plasmon interactions, collisions with outer shell electrons, non-ionizing collisions with inner shell electrons, and radiation losses. FIG. 1B shows a typical “core loss” region 108 of the spectrum. Electron losses in the core-loss region result from ionization of inner shell or “core” electrons and losses are typically greater than 100 eV. The spectra of FIG. 1A and FIG. 1B are not drawn to the same scale; the vertical scale of FIG. 1B is much enlarged compared to FIG. 1A.
FIG. 1A shows a large peak 102, called the “zero-loss peak,” centered on zero energy loss. It is typically about 0.2 eV to 2 eV wide and represents primarily the energy spread in the original beam and small energy losses that occur in predominantly elastic collisions between the beam electrons and atomic nuclei. A broad plasmon peak 104 is caused by a resonance of the beam electrons with the valence electrons. FIG. 1B shows peaks 110, 112, and 114 having much higher energy losses that than those shown in FIG. 1A. Each peak is associated with the removal of a specific inner shell electron and the peak is characteristic of the specific sample material. The core loss spectrum provides information that readily identifies materials present in the sample, although information about the sample is also available from low-loss regions of the energy loss spectrum.
EELS can be performed on a conventional TEM or a Scanning Transmission Electron Microscope (STEM). Whereas in a TEM, the beam is focused by a condenser lens to a small area on the sample, in a STEM, the electron beam is focused to a point and the point is scanned, typically in a raster pattern, across the surface of the sample. FIG. 2A shows a TEM 200 that can perform EELS. Microscope 200 includes an electron source 210 and a focusing column including a condenser lens 212 that projects a beam 213 of primary electrons from source 210 onto a thin sample 214. The beam is composed of high energy electrons, that is, electrons having typical energies of between about 50 keV and 1,000 keV. Electrons that pass through sample 214 enter TEM imaging optics 216. TEM imaging optics 216 can be set to form a magnified image of the sample 214 at the entrance plane of a spectrometer 217, or to form a diffraction pattern at the entrance plane of the spectrometer 217. Electrons 204 pass through entrance aperture 215 and into spectrometer 217, which includes a prism 222 that disperses the electrons according to their energies into different trajectories 224a, 224b . . . 224d, etc.
Electrons are spread vertically according to their energies in an energy dispersive plane 225. A microscope that is capable of operating in the energy selected imaging mode includes an energy-selecting slit 226, having an upper knife edge 226U and a lower knife edge 226L, positioned at or near energy dispersive plane 225. The space between the knife edges is adjustable to pass electrons having energies within different ranges. Electrons 230 that pass through energy-selecting slit 226 are focused by imaging optics 232 onto a detector 234, such as a film, a fluorescence screen, a CCD detector, or an active pixel sensor. Electrons 236 having energies outside the specified range are blocked by energy-selecting slit 226. A spectrum 240 illustrates the quantity of electrons at various energy values, with most electrons being in the “zero-loss peak” 242.
FIG. 2B shows another TEM 248 that can perform EELS. Microscope 248 includes a spectrometer 250 configured as an “in-column” spectrometer, as opposed to spectrometer 217 of FIG. 2A, which is configured as a “post column” spectrometer. In an “in-column” spectrometer, electrons leave the spectrometer parallel to the direction at which the electrons entered. Spectrometer 250 includes for a prism an “omega filter” that typically includes at least four elements 252A, 252B, 252C and 252D. Elements 252A and 252B offset the electron path and disperse the electron beam. Elements 252C and 252D further disperse the electron beam and displace the beam back to the original optical axis. The symmetry between the first half of the omega filter consisting of elements 252A and 252B and the second half of the omega filter consisting of elements 252C and 252D are configured to cause several aberrations of the prisms to cancel. The dispersive actions of these two halves of the omega filter do not cancel and create an energy dispersive plane 254 after element 252D. In this plane, energy-selecting slits 256L and 256R are positioned. Electrons 260 that exit element 252D are focused by imaging optics 232 onto a detector 234.
TEMs commonly use charged-couple device (“CCD”) detectors, which are damaged by high energy electrons. To prevent damage to the CCD detector, TEM detectors include a scintillator that converts the electrons to light, which is then detected by the CCD. The intervening scintillator reduces resolution of the detector. CMOS active pixel sensors (APS), particularly monolithic active pixel sensors (MAPS) were proposed and demonstrated as charged particle detectors for transmission electron microscopy. CMOS MAPS can be used as direct detectors, that is, the electrons impinge directly onto the semiconductor detector, without an intervening scintillator. To reduce backscattering of electrons within in the sensor substrate, which degrades resolution, CMOS MAPS are typically made very thin so that most of the electrons exit the backside of the detector.
As materials science advances, the composition of engineered materials requires tighter control, and it becomes more important to accurately determine the characteristics of the materials. Thus there is a constant demand to improve the resolution of TEM analysis methods, such as EELS, to better characterize materials.